A severe winter storm in central Mexico last month killed hundreds of millions of monarch butterflies. It is the largest number of monarch butterflies killed at one time. However, scientists say the loss is not expected to threaten the species. The butterflies froze to death in two of the largest protected areas for monarch butterflies in Mexico.

The Rosario and Sierra Chincua colonies are in the mountains in the state of Michoacan, west of Mexico City. Most of the butterflies in the two colonies were killed in the storm.

Researchers say the butterflies froze to death after heavy rain fell in the area, followed by freezing temperatures. Monarch butterflies can easily freeze to death if they become wet and the temperature drops to freezing. Scientists say the temperatures following the storm were the lowest recorded in the area in the past twenty-five years.

During the year, monarch butterflies fly long distances, or migrate. They are one of the few kinds of insects to migrate. This has made the monarch popular among nature lovers.

The monarchs spend the winter in Mexico. Each spring, the butterflies fly north after they mate. The females stop to lay their eggs in the southern United States. The adults die soon after. The monarchs that develop from those eggs continue the flight. They return to the same areas in North America where their parents lived. By summer, the butterflies can reach as far north as Canada.

During the autumn, the monarchs return to the same forests in the mountains of Mexico. They like the oyamel tree the best. These tall trees are sometimes completely covered with butterflies.

Some scientists have suggested that the loss of forests in the mountains of Mexico led to the die-off of monarchs last month. They say the remaining forests may no longer provide enough protection to keep the butterflies warm and dry. Every winter, millions of monarchs die in the high mountain forests.

However, scientists note that the species is not in danger of disappearing. That is because there are other, smaller populations of monarchs in the United States that did not migrate to Mexico. Scientists say they will know after more study if the monarch populations in Mexico will be able to recover from the die-off.

This VOA Special English ENVIRONMENT REPORT was written by Cynthia Kirk.